The present invention concerns an amalgam mercury low pressure UV lamp. Such lamps are in operational use. They are used in the disinfection of water and waste water and are superior because of their especially high efficiency. This high efficiency is achieved by binding the mercury into an amalgam (preferably indium) in a lamp with low internal pressure. This mercury is released in the gas phase when the lamp reaches an operating temperature of some 90° C. The decisive factor for this operating temperature is the temperature of the amalgam.
Other types of UV lamps are those low pressure lamps not equipped with amalgam, whose optimal operating temperature is in the region of about 40° C., with these lamps displaying a lesser efficiency, and medium pressure UV lamps which display an internal pressure of some 1,000 mbar and have an operating temperature of several hundred degrees Celsius. These lamps do have a relatively high output in the kilowatt area. But the efficiency, thus the ratio between radiation output in the desired wavelength range and electrical power consumption, is lower than the efficiency of the amalgam mercury low pressure UV lamp found here.
In order to reach the aforementioned optimal temperature of some 90° C. in operation, generic UV lamps are not immersed directly into the liquid to be disinfected. The temperatures prevailing there in the drinking water or waste water domain are too low to maintain the necessary temperature of the UV lamp. Therefore they are sheathed in cladding tubes, which delimit an air gap between their inner wall and the outside of the UV lamp. The low thermal conductivity of air guarantees, in the operation of the UV lamp, that the latter will not cool down to below 90° C. But the basic requirement is a sufficiently high electrical power.
It is apparent from these boundary conditions that generic UV lamps when operated in a cladding tube can only be adjusted to a limited extent. So for example in ex-factory UV lamps, which display an especially high initial UV yield, the electrical power cannot be reduced so far that the radiated UV intensity is reduced to the level released later on when the UV lamp is older. The UV lamp then becomes too cold, which reduces the radiation yield.
In another case, the operating condition may arise where the water to be treated is at a relatively high temperature, for example 60° C. If relatively old UV lamps are used in these operating conditions, a high level of electrical power must be supplied to them in order to provide the necessary UV intensity. The result of the high ambient temperature is that the UV lamp is not cooled to the optimal operating range, as occurs at lower temperatures. The efficiency of the UV lamp falls, because the temperature of the UV lamp settles to over 90° C. This operating condition is not desirable.
It is therefore the task of the present invention to improve an amalgam mercury low pressure UV lamp in such a way that it can be operated independently of the water temperature and preferably with variable UV output.